The maintenance of a predetermined stiffness in aircraft propellers is critical for a number of reasons. Foremost among such reasons are controlling the natural frequencies of the propeller blades in both flatwise and edgewise modes of vibration and to enhance the blades ability to react aerodynamic moments thereon.
It is well known that enhancing the stiffness of a propeller blade increases the natural edgewise and flatwise vibrational frequencies thereof. It is critical that such natural frequencies be of values greater than twice the rotational frequency of the engine powering the propeller, so that engine vibration will contribute minimally to the risk of damage to the propeller blade due to the reinforced natural vibration thereof. Imparting sufficient stiffness to an aircraft propeller blade, will raise these natural frequencies of vibration to such a safe level.
Factors such as a propeller's angle of attack, wind direction and ground effects can cause airflow into an aircraft propeller to be offset from a direction parallel to the axis of rotation of the propeller. It is well known that such offset air inflow to the propeller causes the application of aerodynamic moments to the propeller blades, about axes perpendicular to the pitch change axes thereof. Such moments can be of such severity as to induce a rocking motion of the blades on the pitch change bearings thereof, thereby increasing localized loading in such bearings and in so doing, risking the disintegration thereof.
There are two well known techniques for imparting sufficient stiffness to aircraft propeller blades. One such technique involves the use of materials in the blades, which, when centrifugally loaded under operating conditions, exhibit a required stiffness. Many prior art steel and aluminum propeller blades have sufficient inherent stiffness when centrifugally loaded, to impart to the blades, the desirable vibrational and structural characteristics noted above. However, modern, lightweight composite propeller blades are not massive enough to achieve sufficient stiffness when centrifugally loaded and, therefore, must rely on radially outward preloading of the blade shank against the pitch change bearings thereof, to achieve requisite stiffness.
It has been the practice to achieve such radially outward preloading of a composite blade against the pitch change bearings therefor, with various mounting arrangements by which the blade is retained within the hub. Such arrangements often involve threaded fasteners, split rings, and the like which define discontinuities and, therefore, locations of stress concentrations in the primary load path through the hub when subjected to the high centrifugal loading which the blade undergoes during operation thereof. Furthermore, such preloading mechanisms may require complicated and time consuming disassembly techniques for blade removal and replacement.